Phosphide-based nanowire for visible and near-infrared miniature photon emitters

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We will develop ultra-small (diameters in the range 0.1 ~ 10 micron) near-IR through green, high efficiency light emitting diodes (LEDs) and lasers using our significant experience in growth, characterisation and device development of phosphorus containing nanowires. These structures have applications as red-green pixels in displays, particularly micro displays for virtual and augmented reality. In addition, we will fabricate in one-growth-step, a two-colour 2D emitter array at a reduced cost than current technology for micro-displays and biological sensing and imaging.In contrast to silicon, III-V semiconductors (e.g. GaAs) efficiently convert electrical energy into light. They form the basis of LEDs (solid-state lighting) and lasers (optical data storage - DVDs etc and optical fibre systems). A current challenge is to produce very small LEDs (diameters of 10 micron or less) for applications in micro-displays. In addition, whilst current technology provides high efficiency blue and red LEDs, the efficiency of green LEDs remains significantly lower. This is known as the 'Green Gap Problem'. Traditional LED technology uses growth on large wafers; individual devices are formed using chemical etching and mechanical cleaving. Producing small devices is hence challenging. In addition, these small devices have a large surface to volume ratio that impacts performance. Our approach uses nanowires which grow as very thin strands (typical diameters ~100 nm) vertically upwards from the initial starting substrate. Each nanowire can form an individual LED, hence providing extremely small size devices. During growth, passivation layers can be added to reduce the deleterious surface effects. As the contact area between the nanowire and substrate is very small, combinations of very dissimilar materials are possible, for example a III-V semiconductor on silicon. This allows the direct integration of the nanowire LED with silicon drive electronics. Nanowires give several other advantages. Their small size allows many more combinations of different semiconductors to be integrated, especially ones with different lattice parameters. For standard LED technology, the strain due to the mismatch between materials can degrade the device performance. However for nanowires, their higher strain tolerance provides access to an increased number of design parameters. Most III-V semiconductors can exist in one of two different crystal structures (zinc blende or wurtzite) but for standard growth only the former occurs. However, nanowires can be grown with the wurtzite structure, which has a modified electronic band structure, giving efficient green emission; this is not possible with the zinc blende form. Hence, nanowires based on phosphorus containing semiconductors (e.g. AlInP) have the potential to solve the green gap problem, providing green LEDs with comparable efficiencies to red and blue ones. Because the diameter of the nanowires can be controlled, and this is one parameter determining the crystal structure, we aim to grow 2D nanowire arrays of alternating zinc blende and wurtzite structure. As each structure emits light at a different wavelength this will give a two-colour 2D array of LEDs in a single-growth-step. Current RGB displays require red, green and blue pixel separate growth, followed by picking and placement into the final display, a complex and expensive process. A two-colour array provides increased information density for applications in augmented reality and arrays with pixel sizes below 1um have a range of applications in biological sensing and imaging. Our approach offers the possibility of reduced cost devices as they are formed using a single growth step.Finally, we will build on our development of small size LEDs to fabricate ultra-small size and low operating power visible lasers, aiming to extend the emission wavelength below the current limit of 635 nm towards yellow and possibly green emission (500-565 nm).